An ultrasonic diagnostic apparatus transmits ultrasonic waves generated by array transducers in an ultrasonic probe to a subject and displays images based on ultrasonic waves reflected from boundary surfaces of acoustic impedance. An ultrasonic diagnostic apparatus is extensively used in function diagnosis and conformation diagnosis of organs because it is easy to acquire diagnostic images.
By the most popular scan manner of the ultrasonic diagnostic apparatus, array transducers arranged in a row are driven electrically and ultrasonic images are acquired. Lately, 3-dimensional image data can be acquired by a 2-dimensional array probe or a mechanical oscillation 1-dimension array probe. Volume rendering images or MPR (multi plane reconstruction) images can be obtained by reconstruction of the 3 dimensional image data.
However, because the many scan lines in the above-mentioned acquisition of the 3-dimensional image data require much more time than 2-dimensional image data because of many scan lines. Therefore, it is very difficult to balance time resolution and space resolution.
One solution for this problem, the multibeam forming method. has been invented and used. With the multibeam forming method, a plurality of scan lines can be simultaneously acquired along a plurality of directions. In this method, delay calculations for signals acquired by one transmitting and receiving are executed more than once and each delay calculation corresponds to a distinctive direction of the scan line. The multibeam forming method is a very effective method for generating a 3-dimensional image data.
However, the multibeam forming method has a defect. By this method, because center axis of the transmitting ultrasonic beam and the receiving ultrasound beam are ordinarily different, not only reduction of transmitting and receiving sensitivity but also strain of ultrasonic beam occurs. In addition, when the distance between the transmitting beam axis and each receiving beam axis is different, transmitting and receiving sensitivity is different in accordance with the directions of the receiving beam. In other words, 3-dimensional image data acquired by the multibeam forming method includes considerable artifacts. This defect is also described in Japanese patent disclosure (kokai) No. 11-118063.
FIGS. 17(a) and (b) are frame formats showing a transmitting beam (continuous line) and receiving beams (dashed line) in the multibeam forming method. The transmitting beam is focused at a predetermined depth. On the other hand, focusing of the receiving beam can be kept narrow along the depth direction by dynamic focus.
In this case, the receiving sensitivity of ultrasound is expressed by multiplication of a transmitting acoustic field and a receiving acoustic field. Then in the focused area of the transmitting beam, the transmitting acoustic field affects the receiving acoustic field. Accordingly, as shown by Btr-1 in FIG. 17(b), beam bending to the center direction is generated. Strain of the image occurs by image generation from transmitting and receiving beams having such beam bending.
FIG. 18 is a view showing a frame format of an acoustic field of transmitting and receiving by multibeam forming. The acoustic pressure at end of a transmitting field is less than the acoustic pressure at center of the field. This phenomenon makes the receiving sensitivity uneven and causes stripe patterns in the ultrasonic image acquired by the uneven field. In addition, the reduction of sensitivity causes deterioration of sharpness of the ultrasound image.
Furthermore, time phase difference is affected by the stripe pattern. Between receiving beams in one multibeam forming, the signal of the receiving beams is received simultaneously. However, between a receiving beam in one multibeam forming and a receiving beam in another multibeam forming, the sets of signals of the receiving beams are received at different times. With such time phase difference, the subject and ultrasound probe cannot be kept completely static. Then, in boundary lines between sets of multibeam forming, the signal is discontinuous. Because such discontinuity appears in every predetermined number of receiving beams, the discontinuity is easy to recognize.